This invention relates to a device for use in a receiver of a multivalued or multilevel digital communication system in decoding a received multivalued signal into a decoded signal, namely, into a sequence of data. The multivalued digital communication system may be a frequency shift keying (FSK) or phase shift keying (PSK) communication system.
Supplied with a multivalued digital signal from a transmitter of the communication system, the receiver produces the received multivalued signal in the manner known in the art. A decoder decodes the received multivalued signal with reference to a plurality of signal discriminating reference voltages or levels. When the reference voltages are fixed and are not adjustable, the received multivalued signal is not correctly decoded because a modulator of the transmitter has a modulation degree which is inevitably subjected to secular change and fluctuates with the ambient temperature and because the received multivalued signal has a similarly varying amplitude. This unavoidably results in an increased error rate.
In order to get rid of such undesirable effects, the decoder comprises circuitry for use in automatically controlling the amplitude of the received multivalued signal to a predetermined value. The circuitry is generally called an automatic level control (ALC) circuit.
In the manner which will later be described more in detail, the automatic level control circuit comprises a voltage controlled variable gain amplifier responsive to a control voltage for giving the predetermined value to the amplitude of a signal which is derived from the received multivalued signal as a gain controlled signal. The variable gain amplifier comprises a voltage controlled variable resistance (VCR) or impedance element. Inasmuch as the voltage controlled variable resistance element ordinarily has nonlinear characteristics, the gain controlled signal is subjected to a considerable amount of distortion depending on the amplitude of the received multivalued signal. Furthermore, the decoder has a dynamic range which is restricted by an operational range of the voltage controlled variable resistance element and fluctuations of the operational range. These have made it imposslbe for a conventional decoder to have an appreciably reduced error rate and to cope with a wide amplitude range of the received multivalued signal.